Physico-chemical and Structural Characterization of Alkali-soluble Lignins from Sugar Beet Pulp

Abstract

Two alkali-soluble lignin fractions were extracted from wax, protein, and pectin free cell wall residues of sugar beet pulp with 10% KOH and 7.5% NaOH both at 15°C for 16 h. The physico-chemical properties and structural features of the two isolated pure lignin fractions (LA) were characterized by UV, FT-IR, and 13C-NMR spectroscopy. Both of the lignin fractions contain rather low amounts of neutral sugars (0.89-1.03%) and pectic substances (1.75-1.80%). Their molecular-average weights ranged between 1830-1920 Da. The results obtained by alkaline nitrobenzene oxidation showed that the two lignins contain large amounts of non-condensed guaiacyl units, together with only a few non-condensed syringyl and p-hydroxyphenyl units. The lignin fraction, isolated by 7.5% NaOH at 15°C for 16h, is mainly composed of β-O-4 ether bonds. The less common β-β, β-5, β-1, and 5-5′ carbon-carbon linkages are also found to be present between the lignin structural units. Further studies suggested that hydroxycinnamic acids, such as p-coumaric and ferulic acids, and galacturonic acid are tightly linked to lignin molecules.